3GPP Long Term Evolution (LTE) & MBMS for LTE
An architecture for Multicast/Broadcast Services (MBMS) is discussed as part of the LTE/SAE standardization within 3GPP (SAE=System Access Evolution). In order to distinguish this evolved service architecture from previous architecture releases it is sometimes also referred to as E-MBMS. Similarly, in order to distinguish from the conventional UMTS Terrestrial Radio Access Network (UTRAN), the LTE network is also referred to as E-UTRAN.
Generally an E-MBMS service might be available in a certain area, which is usually referred to as the MBMS Service Area. This service area might span the entire Public Land Mobile Network (PLMN) of an operator, but might also cover only subset of same, depending on operator's configuration. An example for a PLMN-wide service is news broadcast. In contrast, services like traffic information are typical examples of services that might be only of local interest and therefore appropriate MBMS Service Areas might only cover subsets of the PLMN.
Transmission of E-MBMS in E-UTRAN is either a single-cell transmission or a multi-cell transmission as currently specified in 3GPP TS 36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2”, version 8.2.0 (available at http://www.3gpp.org and incorporated herein by reference).
A multi-cell transmission (Multi-cell PTM) supports combining of MBMS transmission from multiple cells. These types of transmissions are also referred to as Single Frequency Network (SFN) transmissions. Multi-cell transmission is available for cells that support the specific SFN requirements, like a strict time synchronization, in order to transmit the same data at exactly the same time and frequency. All cells that fulfill these requirements are part of the so-called MBSFN Area, which will typically cover parts of the MBMS Service Area with high user density, e.g. city centers.
In contrast single cell transmission (Single-cell PTM) is transmitted only on the coverage of a specific cell. It is used outside the MBSFN Area typically experiencing a low user density. Usually this might be the larger part of the MBMS Service Area.
As discussed in 3GPP Tdoc. R3-070395, “Working Assumptions for MBMS” by Vodafone (available at http://www.3gpp.org/ftp/tsg_ran/WG3_lu/TSGR3—55/docs/), E-MBMS services can be provided in one of two modes:                MBMS Broadcast mode—MBMS services are transmitted everywhere within the MBMS Service Area irrespective of UE location or quantity. The UEs do not need to leave RRC Idle state for MBMS reception.        MBMS Enhanced Broadcast mode—MBMS services are not transmitted everywhere and UE location and quantity may be taken into account by the network. The UEs may need to leave RRC Idle state for MBMS reception.        
The presently discussed architecture for E-MBMS services may be found in 3GPP TS 23.401, “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access”, version 1.2.1 (available at http://www.3gpp.org and incorporated herein by reference) and specifies several logical entities each fulfilling specific functions related to MBMS service provision. There might be several options how these logical entities integrate with the entities specified for the LTE/SAE system. It might even be the case that the entities of the E-MBMS architecture are partly or completely separated from the entities of the LTE/SAE system. An exemplary E-MBMS architecture will be described below with respect to FIG. 1.
On the highest level, as part of the mobile network operator's service network, resides the so-called Broadcast/Multicast Service Centre (BM-SC) which is referred to as the eBM-SC for the E-UTRAN. The eBM-SC constitutes the entry point or source for content provider E-MBMS transmissions. Other main functions of the eBM-SC comprise authorization and control of E-MBMS services, for example sending of Session Start or Session Stop indications. Further it may also provide service announcement information to the interested users. Optionally the eBM-SC may also be part of a service provider's network that is different from the mobile network provider. In such a case there would typically exist some special contract relation between the two providers where the service provider provides the content and the mobile network provider provides the means for distribution of the content.
Mobile communications networks are typically separated on a logical level into a core network (CN) and a radio access network (RAN). The core network hosts common functions like authentication, authorization, and accounting (AAA) while the radio access network provides technology-dependent functions like the air interface.
An entity in the core network responsible for handling E-MBMS services might be called E-MBMS gateway (E-MBMS GW). Its main function is the termination of the interface to the eBM-SC and the distribution of the user plane (U-plane) packets for the E-MBMS service in the mobile communications network. With respect to the above-mentioned SFN transmission mode, this might also include the provision of a protocol allowing synchronized transmission on the air interface by the radio access network (SYNC protocol).
In addition to these U-plane functions, the E-MBMS GW might also comprise control planet (C-plane) functions related to E-MBMS services. For example the E-MBMS GW might handle session control signaling between the core network and the radio network like e.g. Session Start or Session Stop signaling. It should be noted that the E-MBMS GW is a logical entity and the described U-plane and C-plane functionality might be integrated into the same node or it might be separated into different nodes, which are connected by a logical interface. For example, 3GPP TS 23.401 refers to the MBMS 1 and MBMS 2 entity, providing the C-plane and U-plane functions described above, respectively.
Another entity within the core network regarding the E-MBMS architecture might become the entity handling user mobility. With respect to the LTE/SAE architecture this entity is referred to as the mobility management entity (MME). As the name implies, its main functionality is mobility management of the UEs. This is of particular importance as the UEs might be in an IDLE state while receiving E-MBMS services. Mobility of UEs in this state is typically tracked only on a coarser granularity at the CN level compared to the cell-accurate tracking of UEs in ACTIVE state at the Radio Access Network (RAN) level.
As already mentioned before, in order to achieve a SFN transmission mode, the same data has to be sent in multiple cells at exactly the same time and frequency. Allocation of time and frequency resources, which are sometimes called a resource blocks, is the typical functionality of radio resource management (RRM). As for SFN transmission the same RRM allocation has to be used for several cells, this functionality might be performed in a separated logical entity which is referred to as the Multi-cell Coordination Entity (MCE) in the LTE/SAE architecture. Again there might be several options where this entity is physically located. It might be a specialized stand-alone entity, either in the RAN or in the CN. Alternatively, it might also be integrated with another node in the RAN or CN. For example, the MCE might be part of the radio base stations serving the cells. In another example, the MCE might be integrated into the operator's Operation & Maintenance (O&M) platform.
Finally on the lowest level, as part of the radio access network, there are several (radio) base stations provided. In an LTE/SAE system these entities are called eNodeBs (eNB). They comprise C-plane and U-plane functions related to E-MBMS services. The main function regarding the U-plane is of course the transmission of the E-MBMS service data into the cell. For example in an SAE/LTE system, the service data is typically transmitted on a logical channel, like the MBMS Transport Channel (MTCH). Depending on the transmission mode (i.e. single-cell or multi-cell transmission) synchronization and RRM information exchange between the eNode Bs and the MCE may be provided to achieve SFN transmission or the Node Bs may schedule transmissions on their own in case of single-cell transmission. A further function of the eNodeBs is the termination of the U-plane interface towards the network (for example E-MBMS GW). The interface towards the network may utilize unicast or multicast in order to distribute the service data to the eNBs.
Typically the U-plane is controlled and configured by the C-plane functions. Regarding the E-MBMS architecture, the handling of Session Start signaling is one example of C-plane function. In case multicast is utilized, the Session Start signaling might inform the eNode Bs which IP multicast address is used for the service data so they can join the transmission. Other C-plane functions that might be comprised in the base stations are to support E-MBMS service reception at IDLE mode UEs. As already discussed above, IDLE mode UEs are usually not known to the eNode Bs. In order to inform possibly present UEs about the availability of an E-MBMS service, the base station may transmit some kind of notification on the available services on a common logical channel within their service area (radio cell(s)). For example in a SAE/LTE system, this common logical channel is the Multicast Control Channel (MCCH).
In order to avoid unnecessary allocation of radio resources on the air interface an eNode B might try to find out whether there is at least one user present in its cell that is interested in the E-MBMS service. For example in the LTE system the procedure is called counting (or polling).
Although not directly part of the E-MBMS architecture, but of course closely related to it, UEs are provided to finally receive the distributed MBMS service data. Unlike for unicast services, there is no direct signaling between the individual UEs and the eNode Bs controlling the respective cells required. As described above, it may be possible for a UE to receive a MBMS service in an IDLE state.
From the UE point of view, the basic information required in order to receive a MBMS service is typically called a service announcement. The service announcement is used to distribute to users service related parameters, e.g. service identifiers like Temporary Mobile Group Identity (TMGI). With this information the UEs are able to detect notification of the availability of a desired service in their current cells e.g. on MCCH. For example, if a notified service identifier matches with one contained in one of the stored service announcements at the UE, it will read the details of the notification including the configuration of the corresponding radio bearer allowing the UE to receive the service data in the cell.
The service announcements may be obtained by the UE in several ways. Generally, the service announcement information may be provided in a standardized format. For example 3GPP Release 6 defines a MBMS User Service Description for this purpose. One possibility for providing service announcements is that a user is subscribed to some services provided by the mobile operator (either directly or indirectly from some 3rd party content provider). In this case the mobile operator would also provide an appropriate service announcement. Another possibility may be that the service announcement information is available from some well-know location, e.g. on a web site. Other options might comprise receiving service announcements via Email or messaging services like SMS (Short Message Service) or MMS (Multimedia Messaging Service). It may also be possible that service announcements are part of the contents of a MBMS service, for example a dedicated announcement channel for other MBMS services.
FIG. 2 shows an exemplary, conventional signaling flow in the network to initiate service provision. When receiving Session Start indication from the eBM-SC 101, the E-MBMS GW 102 provides a Session Start message to all eNode Bs 105-108 in the MBMS Service Area (in the example in FIG. 2, the MBMS Service Area corresponds to the radio cells served by eNode Bs 105-108). The Session Start Message may also be sent to the MCE(s) 103 responsible for MBSFN Area(s) (exemplified by the shaded radio cells controlled by eNode Bs 105 and 106) that are part of the MBMS Service Area.
Assuming for example utilization of IP multicast for distribution of the E-MBMS service data to the eNode Bs, the Session Start message would include required information, e.g. IP multicast address for the service and its TMGI, allowing the receiving eNode Bs to join the E-MBMS service data transmission in the network. The eNode Bs 105, 106 located in the MBSFN Area might join the multicast distribution tree in any case. Further, to joining the multicast distribution tree of the MBMS service, the eNode Bs 105, 106 of the MBSFN Area also allocate/reserve the physical resources for transmission of the MBMS data on the MTCH in their radio cells and indicate the MTCH configuration for the MBMS service to potential recipients in their radio cells by a MBMS notification.
The eNBs 107, 108 outside the MBSFN Area might only join if they host interested users. As discussed above, in order to identify potential interested users the eNode Bs might perform a counting procedure. For example they might send a MBMS Counting Request on the MCCH in their cells. UEs in IDLE state receiving this request might send a MBMS Counting Response to the sending eNode B.
In any case the eNode Bs 105-108 will broadcast a MBMS notification in their radio cells that indicates the MBMS service being available. Accordingly, the mobile terminals, such as for example UE 109 moving into the coverage area of eNode B 108, would only request the service, if a MBMS notification in the radio cell(s) of eNode B 108 indicate that the MBMS service is available, but no downlink resources (MTCH configuration) is indicated in the notification.
It becomes apparent in above example signaling flow that the E-MBMS GW 102 has to distribute Session Start signaling to each eNode B 105-108 in the entire MBMS Service Area. Depending on the size of this area it might comprise many eNode Bs. For example considering E-MBMS services like Mobile TV, the MBMS Service Area might be equal to the entire PLMN of an operator.
Due to the nature of signaling messages, they have to be transmitted in a reliable manner. In order to achieve this, typically a connection-oriented transport protocol is used to convey the messages. For example the LTE/SAE system utilizes the Stream Control Transmission Protocol (SCTP) ensuring reliable, in-sequence transport of signaling messages with congestion control.
Being able to exchange signaling messages between an E-MBMS GW 102 and an eNode B means that there exists an interface between them. As stated above the need to provide Session Start signaling to all eNode Bs 105-108 in the MBMS Service Area may require the E-MBMS GW 102 to terminate a high number of interfaces. Utilizing a connection-oriented transport protocol like SCTP, this means E-MBMS GW 102 may have to terminate a high number of connections, e.g. an SCTP connection to each eNode B in the MBMS Service Area.
The MBMS Service Area might comprise one or more MBSFN Areas, which typically experience a high user density, like in city centers. However, the MBSFN Areas typically cover only a small part of the entire MBMS Service Area. Further, the area outside the MBSFN Areas, which covers the larger part of the MBMS Service Area, usually experience a low density of interested users. Otherwise, the operator would most likely have configured it to be part of a MBSFN Area. In any case the E-MBMS GW 102 has to transmit Session Start signaling also to the eNBs outside the MBSFN Areas, as they are also part of the MBMS Service Area. But as there are most likely only few users present, this may lead to the unnecessary transmission of many Session Start messages to these eNBs. Considering for example E-MBMS service like Mobile TV, the required Session Start messages might further increase with the number of provided TV channels.
“Home Zone”—System Design Concepts for 3GPP LTE
Typically, before deploying a cellular mobile communications network, an operator carefully plans its cellular layout taking into account several parameters like geographical constraints or traffic requirements. Although the deployment may change over time, e.g. adding new cells to increase capacity, it can be considered rather static.
On the one hand, this leads to several benefits for the operator. For example, a quasi-static network typically simplifies maintenance and assures performance of the deployment. On the other hand, such a static approach results in an inflexible system, especially from a subscriber's point of view, as the user's perceived service quality depends on the operator's planning. The operator optimizes the system for the general case, e.g. according to peak or average traffic conditions.
However, this system design may not be flexible enough to react on atypical situations, like the individual situation of the subscriber using the operator's network. As an example, the signal strength received at a user equipment (UE) in a cell may correlate to the perceived service quality. A typical cell radius used in a mobile communications network is in the order of some hundred meters to a few kilometers. Depending on the environment covered by the cell, e.g. buildings or trees, and the UE's location within the cell, the signal strength received at the UE is more or less attenuated. For example at an outdoor location close to the base station emitting the radio signals, received signal strength may be close to 100%. However, for an indoor location the signal strength may easily drop to 50% or less.
To improve the reception conditions especially in local areas with high attenuation (e.g. inside buildings) the operator may deploy a small cell with only a limited range. This type of cells is usually referred to as pico cells. The base stations serving the pico cells are “normal” base stations from a functional point of view, but may have limited capabilities like processing power compared to conventional base stations serving the normal (or macro) cells. A train station may be considered as a typical example for a pico cell deployment. The station may be located inside a building or even underground, but there usually is a high density of subscribers trying to access the mobile communications network.
The deployment of pico cells may increase the flexibility to adapt the cell layout of a mobile communications network to more local demands. On the other hand, similar to deploying macro cells, this is based on planning and estimated or measured traffic conditions and can be considered to be a rather static deployment as well.
Another scenario in which local demands are important is the situation at a subscriber's home. In the past, households typically possessed a fixed telephone line (fixed line) connection in addition to one or more mobile phone subscriptions. The fixed line was used to communicate when being at home, where the mobile subscription was used to communicate while being on the move. Typically the fixed line costs where lower compared to the mobile communication. With the advent of the so-called fixed-mobile convergence (FMC) this separation is more and more vanishing. Mobile subscribers do not own an additional fixed line contract and solely use the mobile communications network, even from their home.
Although technically possible, this changes the demands and requirements towards the mobile communications network. On the one hand, the users may expect to receive a similar service at home as they are used from a fixed line connection, which relates to several aspects spanning from service availability to costs. On the other hand, more and more users will access the mobile communications network, which demands higher capacity of the network.
Regarding the costs, some time ago mobile operators started to offer special tariffs when the subscribers are located at home already. This is realized by applying different charging models based on the current cell used by the subscriber/UE. Per subscriber a specific cell or set of cells may be registered constituting an individual “home zone”. As this is based on macro cells the achieved granularity of the “home zone” is rather coarse.
However, a more critical aspect may be the highly increased number of users per cell trying to access the mobile communications network simultaneously. Depending on the actual environment this may not be possible to be solved with network planning. For example considering an urban living area consisting of many multi-storey apartment buildings, a typical macro cell may contain a huge number of mobile subscribers. This may lead to connectivity problems, if many of them try to access the mobile communications network from their home at similar time.
In order to address this problem, the operator may deploy additional macro cells covering the same area increasing the overall capacity of the mobile communications network locally. Alternatively, the operator may consider deploying several pico cells with the same effect. However, in both options the operator risks inefficient utilization of the offered cell capacities during periods in which only few subscribers are located in the area, which is typically the case during daytime in such a scenario.
Above considerations led to a request for devices that could be used similarly to wireless local area network technology, like a WLAN (Wireless Local Area Network) hotspot, for cellular mobile networks providing limited capabilities and a small cell, just enough to cover a subscriber's home. This is typically referred to as femto or home cell, respectively base station. These devices may be owned and deployed by the individual subscribers and connect to the operator's network via a wired or wireless backhaul connection, e.g. DSL (Digital Subscriber Line). This solution provides a very flexible way to address the specific local needs of each individual subscriber. The operator may benefit from such a deployment, as (home) traffic is offloaded from the macro cells. While the subscribers may benefit from assured cell availability (e.g. good signal strength) and possibly better tariffs.
Techniques for femto cells and related problems are actively discussed in standardization bodies like 3GPP. The issue of restricted access to femto cells is among the most important topics. As mentioned above, the femto cell may be owned by the subscriber and use a subscriber's backhaul connection. Therefore, he may want to control or restrict access to the femto cell, e.g. only the owner and other members of his household. This concept is typically referred to as Closed Subscriber Group (CSG) within 3GPP standardization. Similarly, “CSG cell” may be used as a synonymous for “femto cell”. Further, a home base station is usually referred to as Home NodeB (HNB) or Home eNodeB (HeNB) within 3GPP.
MBMS Deployment in CSG Cells
Generally, deployment of MBMS services to CSG cells may encounter similar problems as discussed above with respect to MBMS deployment to non-SFN network areas, in particular with respect to session control signaling. Another potential problem to be considered in the MBMS deployment in CSG cells providing 3GPP access is security. The Home NodeBs are typically not controlled by the network operator of the 3GPP mobile communication system so that their access to the 3GPP core network should be controlled.
Another issue may be inefficient resource utilization for distribution of multicast or broadcast services to user located in CSG cells. At the same time service continuity should be assured in case of CSG cells, also if users move to a CSG cell located outside the actual service area of the MBMS service provided in the macro cells.